Non-oriented electrical steels are widely used as the magnetic core material in a variety of electrical machinery and devices, particularly in motors where low core loss and high magnetic permeability in all directions of the sheet are desired. The present invention relates to a method for producing a non-oriented electrical steel with low core loss and high magnetic permeability whereby a steel melt is solidified as an ingot or continuously slab and subjected to hot rolling and cold rolling to provide a finished strip. The finished strip is provided with at least one annealing treatment wherein the magnetic properties develop, making the steel sheet of the present invention suitable for use in electrical machinery such as motors or transformers.
Commercially available non-oriented electrical steels are typically broken into two classifications: cold rolled motor lamination steels (“CRML”) and cold rolled non-oriented electrical steels (“CRNO”). CRML is generally used in applications where the requirement for very low core losses is difficult to justify economically. Such applications typically require that the non-oriented electrical steel have a maximum core loss of about 4 watts/pound (about 9 w/kg) and a minimum magnetic permeability of about a 1500 G/Oe (Gauss/Oersted) measured at 1.5 T and 60 Hz. In such applications, the steel sheet is typically processed at a nominal thickness of about 0.018 inch (about 0.45 mm) to about a 0.030 inch (about 0.76 mm). CRNO is generally used in more demanding applications where better magnetic properties are required. Such applications typically require that the non-oriented electrical steel have a maximum core loss of about 2 watts/pound (about 4.4 W/kg) and a minimum magnetic permeability of about 2000 G/Oe measured at 1.5 T and 60 Hz. In such applications, the steel sheet is typically processed to a nominal thickness of about 0.006 inch (about 0.15 mm) to about 0.025 inch (about 0.63 mm).
Non-oriented electrical steels are generally provided in two forms, commonly referred to as “semi-processed” or “fully-processed” steels. “Semi-processed” infers that the product must be annealed before use to develop the proper grain size and texture, relieve fabrication stresses and, if needed, provide appropriately low carbon levels to avoid aging. “Fully-processed” infers that the magnetic properties have been fully developed prior to the fabrication of the sheet into laminations, that is, the grain size and texture have been established and the carbon content has been reduced to about 0.03 weight % or less to prevent magnetic aging. These grades do not require annealing after fabrication into laminations unless so desired to relieve fabrication stresses. Non-oriented electrical steels are predominantly used in rotating devices, such as motors or generators, where uniform magnetic properties are desired in all directions with respect to the sheet rolling direction.
The magnetic properties of non-oriented electrical steels can be affected by thickness, volume resistivity, grain size, chemical purity and crystallographic texture of the finished sheet. The core loss caused by eddy currents can be made lower by reducing the thickness of the finished steel sheet, increasing the alloy content of the steel sheet to increase the volume resistivity or both in combination.
In the established methods used to manufacture non-oriented electrical steels, typical but non-limiting alloy additions of silicon, aluminum, manganese and phosphorus are employed. Non-oriented electrical steels may contain up to about 6.5 weight % silicon, up to about 3 weight % aluminum, carbon up to about 0.05 weight % (which must be reduced to below about 0.003 weight % during processing to prevent magnetic aging), up to about 0.01 weight % nitrogen, up to 0.01 weight % sulfur and balance iron with other impurities incidental to the method of steelmaking.
Achieving a suitably large grain size after finish annealing is desired for optimum magnetic properties. The purity of the finish annealed sheet can have a significant effect on the magnetic properties since presence of a dispersed phase, inclusions and/or precipitates may inhibit normal grain growth and prevent achieving the desired grain size and texture and, thereby, the desired core loss and magnetic permeability, in the final product form. Also, inclusions and/or precipitates during finish annealing hinder domain wall motion during AC magnetization, further degrading the magnetic properties in the final product form. As noted above, the crystallographic texture of the finished sheet, that is, the distribution of the orientations of the crystal grains comprising the electrical steel sheet, is very important in determining the core loss and magnetic permeability in the final product form. The <100> and <110> texture components as defined by Millers indices have higher magnetic permeability; conversely, the <111> type texture components have lower magnetic permeability.
Non-oriented electrical steels are differentiated by proportions of additions such as silicon, aluminum and like elements. Such alloying additions serve to increase volume resistivity, providing suppression of eddy currents during AC magnetization, and thereby lowering core loss. These additions also improve the punching characteristics of the steel by increasing the hardness. The effect of alloying additions on volume resistivity of iron is shown in Equation I:p=13+6.25(% Mn)+10.52(% Si)+11.82(% Al)+6.5(% Cr)+14(% P)  (I)where p is the volume resistivity, in μΩ-cm, of the steel and % Mn, % Si, % Al, % Cr and % P are, respectively, the weight percentages of manganese, silicon, aluminum, chromium and phosphorus in the steel.
Steels containing less than about 0.5 weight % silicon and other additions to provide a volume resistivity of up to about 20 μΩ-cm can be generally classified as motor lamination steels; steels containing about 0.5 to 1.5 weight % silicon or other additions to provide a volume resistivity of from about 20 μΩ-cm to about 30 μΩ-cm can be generally classified low-silicon steels; steels containing about 1.5 to 3.0 weight % silicon or other additions to provide a volume resistivity of from about 30 μΩ-cm to about 45 μΩ-cm can be generally classified as intermediate-silicon steels; and, lastly, steels containing more than about 3.0 weight % silicon or other additions to provide a volume resistivity greater than about 45 μΩ-cm can be generally classified as high-silicon steels.
Silicon and aluminum additions have detrimental effects on steels. Large silicon additions are well known to make steel more brittle, particularly at silicon levels greater than about 2.5%, and more temperature sensitive, that is, the ductile-to-brittle transition temperature may increase. Silicon may also react with nitrogen to form silicon nitride inclusions that may degrade the physical properties and cause magnetic “aging” of the non-oriented electrical steel. Properly employed, aluminum additions may minimize the effect of nitrogen on the physical and magnetic quality of the non-oriented electrical steel as aluminum reacts with nitrogen to form aluminum nitride inclusions during the cooling after casting and/or heating prior to hot rolling. However, aluminum additions can impact steel melting and casting from more aggressive wear of refractory materials and, in particular, clogging of refractory components used to feed the liquid steel during slab casting. Aluminum can also affect surface quality of the hot rolled strip by making removal of the oxide scale prior to cold rolling more difficult.
Alloying additions to iron such as silicon, aluminum and the like also affect the amount of austenite as shown in Equation II:γ1150° C.=64.8−23*Si−61*Al+9.9*(Mn+Ni)+5.1*(Cu+Cr)−14*P+694*C+347*N  (II)
where γ1150° C. is volume percentage of austenite formed at 1150° C. (2100° F.) and % Si, % Al, % Cr, % Mn, % P, % Cr, % Ni, % C and % N are, respectively, the weight percentages of silicon, aluminum, manganese, phosphorus, chromium, nickel, copper, carbon and nitrogen in the steel. Typically, alloys containing in excess of about 2.5% Si are fully ferritic, that is, no phase transformation from the body-center-cubic ferrite phase to the face-centered-cubic austenite phase occurs during heating or cooling. It is commonly known that the manufacture of fully ferritic electrical steels using thin or thick slab casting is complicated because of tendency for “ridging”. Ridging is a defect resulting from localized non-uniformities in the metallurgical structure of the hot rolled steel sheet.
The methods for the production of non-oriented electrical steels discussed above are well established. These methods typically involve preparing a steel melt having the desired composition; casting the steel melt into an ingot or slab having a thickness from about 2 inches (about 50 mm) to about 20 inches (about 500 mm); heating the ingot or slab to a temperature typically greater than about 1900° F. (about 1040° C.); and, hot rolling to a sheet thickness of about 0.040 inch (about 1 mm) or more. The hot rolled sheet is subsequently processed by a variety of routings which may include pickling or, optionally, hot band annealing prior to or after pickling; cold rolling in one or more steps to the desired product thickness; and, finish annealing, sometimes followed by a temper rolling, to develop the desired magnetic properties.
In the most common exemplary method for the production of a non-oriented electrical steel, a slab having a thickness of more than about 4 inches (about 100 mm) and less than about 15 inches (about 370 mm) is continuously cast; reheated to an elevated temperature prior to a hot roughing step wherein the slab is converted into a transfer bar having a thickness of more than about 0.4 inch (about 10 mm) and less than about 3 inches (about 75 mm); and hot rolled to produce a strip having a thickness of more than about 0.04 inch (about 1 mm) and less than about 0.4 inch (about 10 mm) suitable for further processing. As noted above, thick slab casting methods afford the opportunity for multiple hot reduction steps that, if properly employed, can be used to provide a uniform hot rolled metallurgical microstructure needed to avoid the occurrence of a defect commonly known in the art as “ridging”. However, the necessary practices are often incompatible with or undesirable for operation of the mill equipment.
In recent years, technological advances in thin slab casting have been made. In an example of this method, a non-oriented electrical steel is produced from a cast slab having a thickness of more than about 1 inch (about 25 mm) and less than about 4 inches (about 100 mm) which is immediately heated prior to hot rolling to produce a strip having a thickness of more than about 0.04 inch (about 1 mm) and less than about 0.4 inch (about 10 mm) suitable for further processing. However, while production of motor lamination grades of non-oriented electrical steels has been realized, the production of fully ferritic non-oriented electrical steels having the very highest magnetic and physical quality has met with only limited success because of “ridging” problems. In part, thin slab casting is more constrained because of the amount of and flexibility in hot reduction from the as-cast slab to finished hot rolled strip is more limited than when thick slab casting methods are employed.
For the above mentioned reason, there has been a long felt need to develop a means to produce even the very highest grades of non-oriented electrical steels using which are more compatible with the capabilities afforded by thick and thin slab casting and which are less costly to manufacture.